It is not as simple as federal law 'trumping' state law - congress enacting a law specifically to override a state would be problematic, even if constitutionally solid.
You are not considering the implications of congress interfering in a relatively minor state matter when it refuses to legislate on more pressing state/federal issues (gay marriage, legalising cannabis, etc).
You are not considering the implications of congress interfering in a relatively minor state matter when it refuses to legislate on more pressing state/federal issues (gay marriage, legalising cannabis, etc).